U.S. patent application number 14/361155 was filed with the patent office on 2014-11-13 for bend compensated filter fiber.
This patent application is currently assigned to OFS FITEL, LLC. The applicant listed for this patent is OFS FITEL, LLC. Invention is credited to John M. Fini, James W. Fleming, Jeffrey W. Nicholson, Thierry F. Taunay, Man Yan.
Application Number | 20140334788 14/361155 |
Document ID | / |
Family ID | 48613219 |
Filed Date | 2014-11-13 |
United States Patent
Application |
20140334788 |
Kind Code |
A1 |
Fini; John M. ; et
al. |
November 13, 2014 |
Bend Compensated Filter Fiber
Abstract
An optical fiber includes a core region having a longitudinal
axis. A cladding region surrounds the core region. The core region
and cladding region are configured to support and guide the
propagation of signal light in a fundamental transverse mode in the
core region in the directions of the axis. The fiber has a
bend-induced gradient of its equivalent index of refraction
indicative of a loss in guidance of the mode. At least a portion of
cladding region has a graded index of refraction opposite the
bend-induced gradient. The cladding region is configured to have a
substantially flat equivalent index in response to a bend of the
optical fiber.
Inventors: |
Fini; John M.; (Metuchen,
NJ) ; Fleming; James W.; (Westfield, NJ) ;
Nicholson; Jeffrey W.; (Warren, NJ) ; Taunay; Thierry
F.; (Bridgewater, NJ) ; Yan; Man; (Berkeley
Heights, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
OFS FITEL, LLC |
Norcross |
GA |
US |
|
|
Assignee: |
OFS FITEL, LLC
Norcross
GA
|
Family ID: |
48613219 |
Appl. No.: |
14/361155 |
Filed: |
December 14, 2012 |
PCT Filed: |
December 14, 2012 |
PCT NO: |
PCT/US2012/069814 |
371 Date: |
May 28, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61570403 |
Dec 14, 2011 |
|
|
|
61570389 |
Dec 14, 2011 |
|
|
|
Current U.S.
Class: |
385/124 ;
427/163.2 |
Current CPC
Class: |
G02B 6/028 20130101;
G02B 6/03627 20130101; G02B 6/03655 20130101; G02B 6/0286 20130101;
G02B 2006/12166 20130101; G02B 6/0365 20130101; G02B 6/0283
20130101 |
Class at
Publication: |
385/124 ;
427/163.2 |
International
Class: |
G02B 6/028 20060101
G02B006/028 |
Claims
1. An optical fiber, comprising: a core region having a
longitudinal axis; and a cladding region surrounding the core
region, the core region and cladding region configured to support
and guide the propagation of signal light in a fundamental
transverse mode in the core region in the directions of the axis,
the fiber having a bend-induced gradient in its equivalent index of
refraction, at least a portion of cladding region having a graded
index of refraction opposite the bend-induced gradient of the
mode.
2. The optical fiber of claim 1, wherein the cladding region is
configured to have a substantially flat equivalent index in
response to a bend of the optical fiber.
3. The optical fiber of claim 1, wherein the slope of the graded
index of refraction is substantially negative.
4. The optical fiber of claim 3, wherein the slope of the graded
index of refraction is constant.
5. The optical fiber of claim 3, wherein the slope of the graded
index of refraction comprises a predetermined number of steps.
6. The optical fiber of claim 1, wherein the cladding region
comprises an inner cladding region and an outer cladding region and
the optical fiber further comprises a trench region formed between
the core region and the inner cladding region.
7. The optical fiber of claim 6, wherein the portion of cladding
region having a graded index of refraction opposite the
bend-induced gradient extends over at least a portion of the inner
cladding region.
8. The optical fiber of claim 6, wherein the portion of cladding
having a graded index of refraction opposite the bend-induced
gradient extends to a boundary dividing the inner cladding region
and the outer cladding region.
9. The optical fiber of claim 8, wherein the critical radius is
selected by design.
10. The optical fiber of claim 6, wherein the optical fiber is
configured to guide the mode with substantially no loss at low
wavelengths and to exhibit high loss at large wavelengths.
11. The optical fiber of claim 6, wherein the outer cladding region
and the trench region have the same index of refraction.
12. The optical fiber of claim 6, wherein the material index of
refraction of the fiber as a function of radial position in a
pre-compensated portion of the inner cladding region equals the
mode effective index minus a compensation term.
13. The optical fiber of claim 6, wherein the outer cladding region
is formed of at least one glass tube or of pure silica.
14. The optical fiber of claim 13, wherein at least a portion of
the outer cladding region is formed of one tube of Heraeus F300
glass.
15. The optical fiber of claim 6, wherein the trench region is
formed of pure silica or at least one glass tube.
16. The optical fiber of claim 1, wherein the core region includes
at least one dopant that increases its index above that of the
cladding region.
17. The optical fiber of claim 16, wherein the at least one dopant
is at least one of Ge, Al and P.
18. The optical fiber of claim 1, wherein the core region includes
at least one dopant that decreases its index above that of the
cladding region.
19. The optical fiber of claim 18, wherein the one at least one
dopant is F.
20. The optical fiber of claim 1, wherein the core region includes
at least one dopant that renders the optical fiber
gain-producing.
21. The optical fiber of claim 20, wherein at least a portion of
the core region is doped with a rare earth element or Cr.
22. The optical fiber of claim 1, wherein a cross-section of the
core region is one of circular, annular, elliptical, polygonal, or
other more complex shapes.
23. A method of manufacturing an optical fiber, comprising the
steps of: forming a core region having a longitudinal axis; and
forming a cladding region surrounding the core region, the core
region and cladding region configured to support and guide the
propagation of signal light in a fundamental transverse mode in the
core region in the directions of the axis, the fiber having a
bend-induced gradient in its equivalent index of refraction, at
least a portion of cladding region having a graded index of
refraction opposite the bend-induced gradient of the mode.
24. The method of claim 23, wherein the optical fiber is fabricated
using an MVCD deposition method or a stacking method.
25. The method of claim 23, further comprising the step of doping
the core region with at least one dopant.
26. The method of claim 25, wherein the at least one dopant is at
least one of Ge, Al, P, F, a rare earth element, and Cr.
27. The method of claim 1, wherein the effective are of the signal
mode is at least 100 square microns.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. provisional
patent application No. 61/570,389 filed Dec. 14, 2011 and U.S.
provisional patent application No. 61/570,403 filed Dec. 14, 2011,
the disclosures of which are incorporated herein by reference in
their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates generally to optical fibers. More
specifically, the invention relates to a large mode area fiber
employed as an optical filter and exhibiting a refractive index
profile that mimizes bend-induced limitations on the fiber's
filtering properties.
[0004] 2. Discussion of the Related Art
[0005] In the field of optical fiber-based technology, there is an
increasing interest in the use of large mode area fibers, including
fiber-based optical filters, in which it is desirable to filter or
suppress certain wavelengths of light. Such large mode area filter
fibers are known to overcome various nonlinear impairments. For
example, in some fiber amplifiers and lasers, stimulated Raman
scattering (SRS) may limit power and efficiency. Suppressing
wavelengths associated with SRS has been demonstrated to
significantly improve the performance of such amplifiers and
lasers. Some amplifiers may also require suppression of amplified
spontaneous emission or spurious lasing at one wavelength (e.g.
1060 nm) in order to provide efficient amplification of a
wavelength with competing or intrinsically lower gain (e.g. 940
nm). Similarly, significant benefits of wavelength filtering have
been demonstrated in applications such as telecommunications,
sensing, etc.
[0006] In large mode area filter fibers, there is typically a
tradeoff between degree of filtering, effective area, and bend
loss, e.g., a required degree of filtering becomes more difficult
to achieve as mode area increases. Other properties (beam quality,
pump absorption, birefringence, etc.) may also play an important
role in overall performance, and limit the degree of filtering.
[0007] Bend perturbations play an important role in determining
both the bend loss and the degree of filtering. For bend radii
typically required for reasonable handling and packaging of
conventional filter fibers in operation, the bend perturbation
significantly degrades achievable performance.
[0008] When conventional filter fibers are employed in high power
amplifiers and lasers, for example, overall system performance
(output power, pulse energy, etc.) is limited by a failure to
attain large effective area, low bend loss, and strong filtering
when the filter fiber is arranged in a practical coil size.
[0009] At least two different approaches have been employed to
minimize bend-induced losses in optical filter fibers. In one
approach, a composite filter fiber is kept substantially straight
by means of the incorporation of rod-like fibers that are extremely
bend resistant. By forcing the filter fiber to remain essentially
linear, bend-induced loss can be significantly reduced. This
approach may be impractical for many applications, especially when
fiber lengths are on the order of one meter or more. That is, this
approach may achieve large area, low loss, and strong filtering,
but fail to meet constraints on fiber arrangement needed for some
applications.
[0010] A second approach is associated with predetermining a fixed
bend loss by defining a specific "coiling" to be used (consistent
with packaging and other practial constraints on fiber
arrangement), and then utilizing the filter fiber in accordance
with the specified coiling radius (and number of turns). This
approach is subject to bend-induced limitations discussed above,
for example the tradeoff between mode area, bend loss, and
filtering. Similarly, this approach is considered to limit
applications of large area filter fibers, as well as limit
modifications in field implementations and variations in the use of
filter fibers.
[0011] Thus, a need remains in the art for a large mode area filter
fiber that simultaneously attains large effective area, low bend
loss, and strong filtering when the fiber is arranged in a
practical coil size.
BRIEF SUMMARY OF THE INVENTION
[0012] The above-described problems are addressed and a technical
solution achieved in the art by providing an optical fiber that
includes a core region having a longitudinal axis. A cladding
region surrounds the core region. The core region and cladding
region are configured to support and guide the propagation of
signal light in a fundamental transverse mode in the core region in
the directions of the axis. The fiber has a bend-induced equivalent
index of refraction, this gradient in the cladding region having a
strong influence on loss and filtering properties of the mode. At
least a portion of cladding region has a graded refractive index
opposite that of the bend-induced gradient. The cladding region is
configured to have a substantially flat equivalent index in
response to a bend of the optical fiber.
[0013] In an embodiment, the slope of the graded index of
refraction (when the fiber is unbent) may be substantially
negative. The graded index of refraction may be linear, that is
having a substantially constant slope. In an embodiment, the slope
of the graded index of refraction comprises a predetermined number
of steps.
[0014] In one embodiment, the cladding region comprises an inner
cladding region and an outer cladding region and the optical fiber
further comprises a trench region formed between the core region
and the inner cladding region. The portion of cladding region
having a graded index of refraction opposite the bend-induced
gradient may extend over at least a portion of the inner cladding
region. For a given mode of any fiber, the critical radius for
bending is an important metric of filtering properties. By
introducing a region having an index slope opposite that of the
bend-induced gradient, the proposed strategy allows the critical
radius to be determined by design. For example, the portion of
cladding having a graded index of refraction opposite the
bend-induced gradient can extend to the boundary dividing the inner
cladding region and the outer cladding region,. In such cases, the
critical radius can also extend to this boundary. The optical fiber
is configured to guide the fundamental mode with substantially no
loss at low wavelengths and to exhibit high loss at large
wavelengths.
[0015] In one embodiment, the outer cladding region and the trench
region may have the same index of refraction. The material index of
refraction of the fiber as a function of radial position in a
pre-compensated portion of the inner cladding region may equal the
mode effective index minus a compensation term.
[0016] In another embodiment, the outer cladding region is formed
from at least one glass tube or of undoped silica. The at least one
glass tube may comprise commercially available tubing, such as
Heraeus F300 glass, for example. The trench region may be formed of
undoped silica or at least one glass tube.
[0017] In one embodiment, the core region may include at least one
dopant that increases its index above that of the cladding region.
The at least one dopant may be at least one of Ge, Al and P. In
another embodiment, the core region may include at least one dopant
that decreases the index, which may be F or B. In another
embodiment, the core region may include at least one dopant that
renders the optical fiber to be gain-producing, which may be a rare
earth element or Cr.
[0018] A cross-section of the core region may be circular, annular,
elliptical, polygonal, or other more complex shapes.
[0019] The above-described problems are addressed and a technical
solution achieved in the art by providing a method of manufacturing
an optical fiber, comprising the steps of forming a core region
having a longitudinal axis; and forming a cladding region
surrounding the core region, the core region and cladding region
configured to support and guide the propagation of signal light in
a fundamental transverse mode in the core region in the directions
of the axis, the fiber profile having a bend-induced gradient of
equivalent index of refraction indicative of a loss in guidance of
the mode, at least a portion of the cladding having a graded index
of refraction opposite the bend-induced gradient. In one
embodiment, the optical fiber may be fabricated using an MVCD
deposition method or a stacking method.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The present invention may be more readily understood from
the detailed description of an exemplary embodiment presented below
considered in conjunction with the attached drawings and in which
like reference numerals refer to similar elements and in which:
[0021] FIG. 1 is a schematic diagram of a graded-index core region
of a large mode area filter fiber, according to an embodiment of
the present invention;
[0022] FIG. 2 illustrates an exemplary section of a conventional
large core diameter optical fiber that has been bent to exhibit a
defined bend radius;
[0023] FIGS. 3A and 3B illustrate the impact of a bend on the
refractive index of a conventional non-filtering fiber, with FIG.
3A showing the refractive index profile for a substantially
"straight" section of fiber, and FIG. 3B showing the equivalent
refractive index profile for a fiber as bent in FIG. 2;
[0024] FIG. 4A graphically illustrates filtering loss versus
wavelength for a single conventional filter fiber having an index
profile;
[0025] FIG. 4B is a plot of filtering loss versus effective area
A.sub.eff of several conventional bent filter fibers, each having a
bend radius of about 12 cm;
[0026] FIG. 5A is a refractive index profile of a conventional
straight filter fiber;
[0027] FIG. 5B is an equivalent refractive index profile of a
conventional filter fiber when the filter fiber is bent;
[0028] FIG. 6A is a refractive index profile of a pre-compensated
filter fiber when the filter fiber is straight, according to an
embodiment of the present invention;
[0029] FIG. 6B is an equivalent refractive index profile of a
pre-compensated filter fiber when the filter fiber is bent,
according to an embodiment of the present invention;
[0030] FIGS. 7A and 7B depict additional refractive index profiles
of pre-compensated filter fibers when straight and bent,
respectively, according to an embodiment of the present
invention;
[0031] FIG. 8 shows an illustrative target index profile plotted
along with irregularities in the index profile and a resulting
summed total index profile;
[0032] FIGS. 9A and 9B depict straight and bent profiles,
respectively, of relative refractive index versus distance from the
center of a filter fiber designed to be pre-compensated according
to an embodiment of the present invention;
[0033] FIG. 9C is a plot of filtering loss versus effective area
A.sub.eff of two proposed designs and several conventional bent
filter fibers, each having a bend radius of about 12 cm;
[0034] FIG. 10A illustrates one example of an unconstrained
bend-compensated index profile, according to an embodiment of the
present invention;
[0035] FIG. 10B shows a bend-compensated profile in which
n.sub.out-n.sub.trench is constrained to match n.sub.A-n.sub.B for
available preferred materials A and B, according to an embodiment
of the present invention;
[0036] FIG. 11 shows a pre-compensated filter fiber designed with
an index constraint, where the trench index equals the outer
cladding index; and
[0037] FIGS. 12A and 12B illustrate a rough technique for designing
a bend-compensated profile having an assumed linear gradient in the
inner cladding region of .gamma.n/R.sub.bend, according to an
embodiment of the present invention.
[0038] It is to be understood that the attached drawings are for
purposes of illustrating the concepts of the invention and may not
be to scale.
DETAILED DESCRIPTION OF THE INVENTION
[0039] FIG. 1 is a schematic diagram of a core region of a large
mode area filter fiber 100, according to an embodiment of the
present invention. The filter fiber 100 may include a core region
102 having a longitudinal axis 104 and a cladding region 106
surrounding the core region. The core region 102 and cladding
region 106 may be configured to support and guide the propagation
of signal light in the core region 102 in the direction of the axis
104. To this end, the refractive index of the core region 102
(n.sub.core=n.sub.c) is greater than that of the cladding region
106 (n.sub.clad). Preferably the core region 102 and the cladding
region 106 are configured to propagate signal light preferentially
in a fundamental transverse mode at the center wavelength of the
signal light.
[0040] The term center wavelength of the signal light is intended
to recognize the well-known phenomenon of line broadening; that is,
no signal source emits light at precisely a single wavelength.
Rather, all light sources emit at a center wavelength, where the
intensity is typically maximum, as well as at lower intensities in
a range of wavelengths extending on both sides of the center
wavelength. This range is known as the linewidth. Hereinafter,
signal wavelength is understood to refer to signal light that is
inherently characterized by a non-zero linewidth.
[0041] The filter fiber 100 may be a standard, non-gain producing
fiber used in a variety of applications including, for example,
transmission systems, access systems, sensor apparatus, motor
vehicles, and the like. Alternatively, the filter fiber 100 may be
a gain-producing filter fiber, which finds application in, for
example, fiber optic amplifiers and fiber optic lasers.
[0042] The core region 102 may be a single region, or it may be a
composite of two or more different regions. The core region(s) may
have a gradient in index of refraction and may have, for example,
different dopants, different indices, and/or, in the case of a
gain-producing filter fiber, different optical gains. The cladding
region 106 may be a composite of two or more different regions. As
with the core region, the cladding regions may have, for example,
different dopants and/or different indices. Thus, the cladding
region 106 may comprise an inner cladding region and one or more
outer cladding regions (not shown) disposed radially outside of the
inner cladding region. The outer cladding region may include, for
example, a down-doped region (or trench), which has an index less
than that of the inner cladding region. The index of the inner
cladding region (n.sub.clad) constitutes a frame of reference for
the measurement of other index differences; to with,
.DELTA.n.sub.c=n.sub.core-n.sub.clad, which is often referred to as
the contrast.
[0043] An optical filter fiber 100 may be made of silica and one or
more suitable dopants in particular regions. For example, the core
region 102 may include one or more dopants that increase its index
above that of the cladding region 106, which may comprise
substantially pure or undoped silica. Illustrative index-increasing
dopants include Ge, Al and P. However, for reasons well known to
those skilled in the art, the core region 102 may also include one
or more index-decreasing dopants such as F or B. Likewise certain
portions of the inner cladding region may include one or more
index-decreasing dopants to form trenches. Some regions may include
both index-increasing and index-decreasing dopants.
[0044] If the filter fiber 100 is a gain-producing fiber, then the
core region 102 may include at least one gain-producing dopant
(e.g., a rare earth element or Cr). The gain-producing dopant may
be distributed throughout the entire core region 102, or it may be
confined to only a portion thereof.
[0045] Although the use of the term radius in the foregoing
discussion implies that the cross-sections of the core regions 102
are circular and/or annular, in practice, the core regions 102 may
be non-circular; for example, they may be elliptical, polygonal, or
other more complex shapes. Nevertheless, as is common in the art,
the term radius may be employed for simplicity and clarity.
[0046] FIG. 2 illustrates an exemplary segment of optical fiber 200
that has been bent to exhibit a defined bend radius. As shown, bent
fiber 200 is defined as having a bend radius R.sub.bend, with the
x-y orientation as shown. The bending of a fiber, as mentioned
above, has been found to introduce changes in mode properties,
including loss, filtering and effective area. In particular, the
equivalent index model of the bent fiber 200 can be determined and
then analyzed to account for the different path lengths "seen" by a
propagating optical signal at different transverse positions x as
it travels around the bend of radius R.sub.bend as follows:
( length ) ( angle ) = R bend + x , ##EQU00001##
where path lengths are adjusted by defining the equivalent index
profile n.sub.eq.sup.2,
n eq 2 ( x , y ) = n 2 ( x , y ) ( 1 + 2 x R bend ) ,
##EQU00002##
which is considered to be a modified version of the nominal
refractive index profile (n.sup.2) of the optical fiber material.
FIGS. 3A and 3B illustrates the impact of a bend on the refractive
index of a conventional non-filtering fiber, with FIG. 3A showing
the refractive index profile for a substantially "straight" section
of fiber, and FIG. 3B showing the refractive index profile for a
fiber as bent in FIG. 2. As shown, the equivalent refractive index
is shifted upward along a slope defined by the following
relation:
.DELTA. n = nx R bend . ##EQU00003##
[0047] The equivalent index model defined above leads to the
conclusion that the effect of a bend in a fiber (particularly a
large area fiber) can be likened to adding a constant index
gradient to the profile of the fiber material itself (assuming low
contrast). Prior art fiber design has concentrated on controlling
the filtering properties subject to the limitation that the
equivalent index profile has a slope in the cladding equal to the
bend-induced gradient [that is, that the cladding has constant
material refractive index]. This limitation imposes an important
tradeoff between mode area, filter performance and bend loss. In
accordance with the present invention, however, we consider designs
where the equivalent index profile has a slope much smaller in
magnitude than the bend induced gradient. In such cases, the bend
induced gradient is at least partially cancelled by a slope in the
fabricated index profile. In many high power applications, it is
important to prevent detrimental nonlinear effects from occurring
in the signal light. To this end, fibers with large mode area (LMA)
are often used. A LMA fiber has a relatively large mode-field
diameter (MFD) or a relatively large mode-field area (A.sub.eff).
Those skilled in the art recognize that MFD and A.sub.eff are
equivalent parameters only when the mode field shape is
substantially Gaussian. However, when the mode-field shape departs
from strictly Gaussian, then the MFD is generally not the best way
to describe the size of the guided mode relevant to nonlinearity.
In this case, the industry relies instead upon A.sub.eff, which is
given by:
A eff = ( .intg. E 2 A ) 2 .intg. E 2 A ##EQU00004##
where E is the transverse spatial envelope of the mode's electric
field, and the integrations are understood to be performed over the
cross-sectional area of the fiber. When the mode-field shape is
close to an axisymmetric (i.e., symmetric about the longitudinal
axis of rotation of the fiber) Gaussian function, the MFD is an
appropriate metric for the diameter of the mode and may be
expressed as:
MFD = 2 2 .intg. E 2 A 2 .intg. E r 2 A ##EQU00005##
where r is the radial coordinate. When the mode-field shape is
exactly equal to an axisymmetric Gaussian function, then
A.sub.eff=.pi..times.MFD.sup.2/4.
[0048] Although LMA fibers advantageously reduce the effects of
nonlinearities, they are disadvantageously more sensitive to bend
distortion, which reduces the A.sub.eff and radially displaces and
distorts the optical modes. Mode size can impact other important
system aspects, such as splice loss.
[0049] With conventional filter fibers, there is an inherent
tradeoff between mode area and filter selectivity or sharpness.
FIG. 4A graphically illustrates filtering loss versus wavelength
for a single conventional filter fiber having an index profile
shown in the inset (to be described in more detail below). FIG. 4B
is a plot of filtering loss versus effective area A.sub.eff of
several conventional bent filter fibers, each having a bend radius
of about 12 cm. As shown in FIGS. 4A and 4B, there is a limit (as
represented by the black dashed line) of how much filtering (or
noise wavelength suppression) may be achieved. This limit decreases
with effective area.
[0050] FIG. 5A is a refractive index profile 700 of a conventional
straight filter fiber. FIG. 5B is a refractive index profile 700'
of a conventional filter fiber when the filter fiber is bent. In
the straight fiber profile of FIG. 5A, the profile 700 typically
exhibits a high index of refraction in a core region 702 near the
fiber center, with a rapidly decreasing index in a region 704 that
settles in a substantially low index of refraction region known as
a trench 706, followed by a rapidly increasing index in the region
708 that plateaus in a cladding region 710.
[0051] Ideally, with no purturbations, the conventional filter
fiber of FIG. 5A is configured to rigorously guide a mode of a low
wavelength of signal light with no loss, as illustrated by the
dashed line 714 (i.e., depicting effective index of refraction
n.sub.eff at short wavelengths). At the same time, as illustrated
by the dashed line 712 (i.e., depicting effective index of
refraction n.sub.eff at long wavelengths), the conventional filter
fiber of FIG. 5A is configured to exhibit high loss at large
wavelengths.
[0052] A conventional filter fiber, when subjected to bends as
illustrated in FIG. 5B, becomes leaky, i.e., the filter fiber
equivalent index profile 700' exhibits an "upward tilt" or
bend-induced gradient with increasing radius, which is pronounced
in the cladding region 710. At a critical bend radius 716 in the
cladding region 710, as represented by the intersection of the
dashed line 714 and the tilted profile 700', selectivity is reduced
in that shorter wavelengths are no longer guided.
[0053] The tighter the bend, the closer the critical radius is to
the center of the core region 702, such that selectivity is
diminished for progressively lower wavelengths. This forces such
designs to have smaller effective mode area than desired. Thus,
ideal performance that a conventional filter fiber may achieve in
principle is not achievable in a practical, coiled filter
fiber.
[0054] FIG. 6A is a refractive index profile 800 of a
pre-compensated filter fiber 801 when the filter fiber is straight,
according to an embodiment of the present invention. FIG. 6B is a
refractive index profile 800' of the pre-compensated filter fiber
801' when the filter fiber is bent, according to an embodiment of
the present invention. The pre-compensated filter fiber 801, 801'
includes a core region 802 near the fiber center, and a cladding
region 806 formed about the core region 802. The cladding region
806 is configured to guide a mode supported by the core region 802.
In operation, the cladding region 806 has an index of refraction
designed to induce loss in guidance of the mode at particular
wavelengths. In an embodiment, at least a portion 808 of the
cladding region 806 has a graded-index index of refraction opposite
the effective bend-induced gradient.
[0055] In one embodiment, the cladding region 806 may comprise an
inner cladding region 812 and an outer cladding region 814. A
trench region 816 may be formed between the core 802 and the inner
cladding region 812. FIG. 6A depicts the inner cladding region 812
having a sub-region 808 with a tilt of index of refraction that
pre-compensates an anticipated bend-induced gradient over the
entire extent of the inner cladding region 812.
[0056] Bend loss is most sensitive to the index profile in a region
where the equivalent index of the fiber is close to the effective
index of the mode. Thus bend compensation can be effective even if
only this portion of the fiber profile has a precompensating tilt
as shown in FIGS. 7A and 7B. Referring to FIG. 7A, in one
embodiment, a pre-compensated tilt in the index of refraction 900
of a filter fiber 901 may extend over only a portion 908 of the
inner cladding region 912, preferably including a region at the
outside of the inner cladding. FIG. 7A shows a pre-compensated tilt
902 that is substantially continuous. Referring to FIG. 7B, in an
alternate embodiment, the filter fiber 901' may be fabricated with
a small number of negative-going steps 918. In FIGS. 6A, 7A, 7B,
the graded-index portion 808, 908 of the inner cladding region 812,
912 has a gradient opposite that of the bend-induced gradient.
[0057] Referring again to FIG. 6A, the refractive index profile 800
exhibits a high index of refraction in the core region 802 near the
fiber center, with a rapidly decreasing index in the region 803
that settles in the substantially low index of refraction trench
region 816, followed by rapidly increasing index in the region 807.
In contrast to the filter fiber of FIG. 5A exhibiting a substantial
plateau in a cladding region, the index of refraction of the inner
cladding region 812 of FIG. 6A exhibits a negative-going tilt or
grading to precompensate for an anticipated bend in the filter
fiber 801.
[0058] With no perturbations, the pre-compensated filter fiber 801
of FIG. 6A is configured to rigorously guide a mode of a low
wavelength of signal light with no loss and to exhibit high loss at
large wavelengths. When subjected to bends as illustrated in FIG.
6B, the filter fiber 801 does not become leaky, i.e., the filter
fiber equivalent index profile 800' exhibits a substantially flat
equivalent index of refraction with increasing radius up to a
critical radius 820 at an interface with the outer cladding region
814. Selectivity is thereby maintained in at least a portion of the
inner cladding region 812. The critical radius 820 for
short-wavelength leakage is now determined by the design,.
[0059] In the embodiment depicted in FIG. 6A, the pre-compensated
portion 808 of the filter fiber 801 has an index gradient
dn/dr==-n.sub.substrate/R.sub.eq,
where n.sub.substrate is the nominal index of the filter fiber 801
(e.g., with silica index=1.45), and R.sub.eq is the equivalent bend
radius in the Marcuse model. As shown in FIG. 6B, when the filter
fiber 801 is bent, optical properties are determined by the
equivalent index, which has a substantially flat region 822 and is
therefore more conducive to large effective mode area.
[0060] In one embodiment, R.sub.eq may simply be the physical
radius of a coil of filter fiber. In another embodiment, the
physical coil radius (R.sub.coil) may be adjusted for strain and
other effects to provide an equivalent radius (e.g.,
R.sub.eq=R.sub.coil/0.8). Further, the radius of curvature in a
real fiber is never perfectly constant, and portions of the filter
fiber 801 with smaller radius of curvature may contribute most of
the loss, so R.sub.eq may depend on the detailed fiber handling and
layout in a more complicated way, and may be determined
empirically. In another embodiment, a filter fiber may be designed
for an anticipated arrangement and utilized in a different
arrangement. Pre-compensation may provide significant benefit
within a range of arrangements (e.g., coil sizes), although a large
mismatch between the anticipated and actual curvature may result in
performance degradation (e.g. high bend loss, insufficient
filtering, multi-modedness, etc).
[0061] In another embodiment, the material index of refraction of
the filter fiber as a function of radial position r in the
pre-compensated portion of the inner cladding region 812, 912
approximately equals the mode effective index n.sub.eff minus a
compensation term, or:
n.sub.portion.about.n.sub.eff(1-r/R.sub.eq)
where Req is less than 50 cm and where the extent of the
pre-compensated portion is large, illustratively greater than 20
microns.
[0062] The index of refraction of the core region 802, the radius
of the core region 802, and index of refraction of the trench
region 816 together substantially determine the effective area
A.sub.eff and the effective refractive index n.sub.eff of a
fundamental mode at a signal wavelength (where low loss is
desirable) and at a noise wavelength (where large suppression is
desirable). The equivalent index of the cladding region 806
(including a compensating tilt and bend perturbation) relative to
the fundamental mode effective index determines the loss at the
signal and noise wavelengths, respectively. In a preferred
embodiment, the equivalent index of the cladding region 806 should
fall below the signal-wavelength index
n.sub.eff(.lamda..sub.signal) but also fall above the noise index
n.sub.eff(.lamda..sub.noise) over the largest possible portion of
the cladding region 806. As shown in FIG. 6B, this is best achieved
when the bend perturbation is pre-compensated (and can be achieved
to some extent even if the bend perturbation is imperfectly
compensated). In some preferred embodiments, the pre-compensated
filter fiber 801 includes a cladding region 806 with large radial
extent that has a significant negative index gradient.
[0063] An actual fabricated profile may have significant
irregularities as would be appreciated by one of ordinary skill in
the art having the benefit of this disclosure. FIG. 8 shows an
illustrative target index profile 1000 plotted along with
irregularities in the index profile 1002 and a resulting summed
total index profile 1004. The gradient of the total index profile
1004 deviates significantly from the gradient of the target index
profile 1000, yet calculated performance of the summed total index
profile 1004 is very good. It will be appreciated by one of
ordinary skill in the art having the benefit of this disclosure
that the trend of the gradient or a smoothed gradient matches the
target gradient over a large cladding region.
[0064] In an example, as shown in FIG. 8, the gradient of the
target index profile may have a pre-compensated region with radial
extent >40 microns.
[0065] FIGS. 9A and 9B depict straight and bent profiles,
respectively, of relative refractive index versus distance from the
center of a filter fiber designed to be pre-compensated according
to an embodiment of the present invention. In the example shown,
A.sub.eff is selected to be about 150 .mu.m.sup.2 with a greater
than 10.times. more selectivity than conventional W-fibers, as
shown in FIG. 9C. The effective index difference is selected to be
about n.sub.eff(1060 nm)-n.sub.eff(1135
nm).about.0.6.times.10.sup.-4. The fiber is selected to be radially
symmetrical. The example of FIGS. 8A-8C illustrates that different
signal loss thresholds may be employed, e.g., 0.1 dB/m. The
threshold can ultimately be chosen to suit the anticipated fiber
length and acceptable signal loss.
[0066] Similarly, the degree of suppression of the noise wavelength
depends on the application and system parameters. Typically fiber
length for gain fibers may be a few meters (e.g. for core-pumped Yb
fibers with strong pump absorption) or may be many tens of meters
(e.g. for cladding-pumped Er-doped fibers). For non-gain-doped
fibers, length can vary over a much wider range, including lengths
much shorter than 1 m. The total amount of noise loss (equal to the
noise loss rate times the length) should be at least a few dB, but
preferably greater than 20 dB. For example, the noise loss rate is
thus preferably greater than 10 dB/m for short fiber lengths (e.g.
2 m), in order to achieve a product of 20 dB. For longer lengths
(e.g. 20 m), 1 dB/m loss is sufficient to induce large total loss.
For gain fibers, the suitable rate of signal loss and noise loss
can be specified relative to the pump absorption (both specified in
dB/m): noise loss should be at least 20% of pump absorption, and
preferably at least 100% of pump absorption. Signal loss should be
less than 5% of pump absorption, and preferably less than 1% of
pump absorption.
[0067] The current invention is intended primarily for effective
areas too large to achieve by more conventional means. Accordingly,
it is primarily intended for effective areas greater than 50 sq.
microns, and especially greater than 100 sq. microns.
[0068] A very precisely defined index profile is desirable to
achieve best performance. Preferred fabrication methods can improve
index profile precision, make fabrication easier, or reduce cost.
In an embodiment, bend-compensated filter fibers may be fabricated
by conventional deposition (e.g. MCVD) or by "microstructured"
fabrication methods (stacking). Such considerations and
manufacturing methods may impose constraints on a design. FIG. 10A
illustrates one example of an unconstrained bend-compensated index
profile, while FIG. 10B shows a bend-compensated profile in which
n.sub.out-n.sub.trench is constrained to match n.sub.A-n.sub.B for
available preferred materials A and B. Other parameters (e.g.,
trench width) may be adjusted to satisfy the design requirements
with this constraint.
[0069] In another example, the outer cladding may be formed in part
from pure silica or a commercially available glass tube.
Preferably, tubes with an index of pure silica or slightly above
(0-5e.sup.-4) may be used, since these have low cost and highly
predictable index. Other regions may be formed of pure silica or
commercially available tubes. For example, the trench region may be
pure silica.
[0070] The index difference between two regions may be constrained
by available preferred materials. For example, two regions may be
formed of identical materials, and have zero index difference,
e.g., n.sub.out-n.sub.trench.apprxeq.0. In another example, the
well known fact that the refractive index of nominally pure and
undoped silica can vary depending on fabrication conditions and
method due to impurities and defects can be exploited by using one
tube of Heraeus F300 glass to form part of the outer cladding,
while the trench may be formed of undoped silica derived from
another source, such as vapor phase deposition. This can result in
an index difference, e.g.,
n.sub.out-n.sub.trench.apprxeq.5.times.10.sup.-4.
[0071] In another example, a pre-compensated filter fiber may be
designed with an index constraint as shown in FIG. 11, where the
trench index equals the outer cladding index.
[0072] In an example, as shown in FIG. 11, the gradient of the
target index profile may have a pre-compensated region with radial
extent >30 microns.
[0073] FIGS. 12A and 12B illustrate a rough technique for designing
a bend-compensated profile having an assumed linear gradient in the
inner cladding region of .gamma.n/R.sub.bend. The graded-index
needed to pre-compensate a bend in the inner cladding region may be
selected such that .DELTA.n/.DELTA.r.about..gamma.n/R.sub.bend and
.gamma.-1(.gamma..about.0.8). In one example, R.sub.bend=12 cm,
.gamma..about.0.8, gradient.about.0.1/cm.about.10.sup.-5/.mu.m. A
simple construction provides an estimate of how much relative
leakage can be achieved between two wavelengths (.lamda..sub.signal
and .lamda..sub.noise). As shown in FIG. 11B, a rough estimate of
relative leakage rate is:
.kappa.=(2.pi./.lamda.)(2n.DELTA.n.sub.eff).sup.1/2 [0074]
Field.about.exp(-.DELTA..kappa.r) [0075] Relative
leakage.about.exp(-2(2.pi./.lamda.)(2n.DELTA.n.sub.eff).sup.1/2.DELTA.r)
[0076] Relative leakage.about.exp(-2.kappa..DELTA.r)
[0077] For example, if .DELTA.r.about.50 microns and
.DELTA.n.sub.eff.about.0.6.times.10.sup.-4, then the relative
leakage is .about.4.times.10.sup.-4.
[0078] It is to be understood that the exemplary embodiments are
merely illustrative of the invention and that many variations of
the above-described embodiments may be devised by one skilled in
the art without departing from the scope of the invention. It is
therefore intended that all such variations be included within the
scope of the following claims and their equivalents.
* * * * *